In automobiles such as passenger cars, trucks, and buses, noise is caused in an engine room while an engine is operating. Such noise is transmitted from the inside of the engine room to the outside and inside of a vehicle compartment.
Patent Document 1 discloses a sound absorbing material attached to a lower surface of a hood panel to absorb engine noise of an automobile and made of a porous base material. The porous base material is made of organic fibers such as polyester fibers or polyethylene fibers, or is made of a plastic foaming body with an open-cell structure, such as a polyester foaming body or polyethylene foaming body.
However, such a sound absorbing material is typically effective to noise outside a vehicle compartment, i.e., noise in a frequency band with a relatively-high frequency of equal to or higher than 1000 kHz. However, the sound absorbing material is less effective to noise inside the vehicle compartment, i.e., noise in a low frequency band of equal to or lower than 1000 Hz.
Thus, Patent Document 2 discloses a soundproof engine hood configured to reduce noise inside a vehicle compartment by a sound absorbing effect of a resonance sound absorbing pipe provided in an engine hood. Moreover, Patent Document 3 discloses a muffling device of an internal combustion engine configured such that a resonance muffling chamber communicating with an air intake path of the engine is provided in a hollow reinforcing frame attached to a rear surface of an engine hood.